White spaces, angry faces: Inside the battle over ‘interference’

Spectrum sensing

To deal with interference issues, current proposals on the table could require white space devices to use three separate techniques to identify open spectrum before broadcasting: spectrum sensing, geolocation, and beacons. This tripartite approach wasn't initially favored by white space backers, but companies like Google and Philips and Microsoft have been willing to compromise in the hope of dialing back the opposition from groups like wireless microphone manufacturers.

Spectrum sensing was the first proposed solution and is simple enough to understand: The white space device would scan the airwaves before transmitting. It would only broadcast on a channel found to be empty, assuming that the spectrum sensing is accurate. The NAB worries that it might not be.

The reasons are threefold. First, there's the possibility of bad engineering, in which the device in question simply isn't properly sensitive to TV signals (especially distant, low-power ones). This is what the FCC's Office of Engineering Technology has been testing in its labs and in the field. Results from the first round of testing last year were promising, but problems remained.

Some of those problems were later attributed to broken prototype devices, which brings us to the second broadcaster concern: reliability. It's all fine and good if the devices work when they're released, but what happens when devices break or have "power issues" like the prototypes in question did? The FCC is not testing device reliability; in fact, it isn't even working with production-ready devices.

The third concern is that, even if everything works, interference might still result due to things like the "hidden node" problem.

Arthur, Merlin, and the case of the hidden node

Imagine two hills, separated by a valley. On top of the left hill is a TV transmitter, and on top of the right hill, a man watching football on his TV. We'll call him Arthur. In the valley between them lies the home of Arthur's nemesis, a white space devices user named Merlin.

Arthur's TV reception is great, except when Merlin turns on his white space box to get himself some hot wireless broadband action. The box works perfectly, it uses spectrum sensing, but it still causes problems. Why?

Because the hills attenuate the signal. When Merlin's box scans the airwaves, it sees nothing because it's in the valley, and the TV signal falls below its detection threshold. It transmits on the same band that the TV station uses, and when it does so, Arthur's football game gets scrambled. The same thing could happen more easily in an urban canyonland like Manhattan.

Lynn Claudy, of the NAB, says that problems like hidden nodes mean that spectrum sensing is simply based on "a false premise." Just because a channel is clear at the moment, and from a particular location, doesn't mean that other viewers won't be affected by a broadcast on that frequency.

Wally is about to be flagged for interference

Of course, this is the point at which the debate becomes contested once more. Ed Thomas says that the hidden node issue is real, but the only thing a hidden node does is create a lower-level signal. One way of addressing the issue, which white space devices do, is to bake in 30dB of margin.

The threshold for digital TV visibility is -84dBm; below that power, no signal can be displayed. But white space devices detect the presence of signals all the way down to -114dBm, and Thomas says that the huge cushion of margin will take care of hidden node issues. The devices could potentially do sensing at even lower power levels, but at that point, the noise starts to generate false positives.

New America's Meinrath also points out that the transmitting and receiving nodes on a white space network need to indicate a clear channel before data transmissions will begin. Even if one node is located in a hidden node, the other node it must connect with probably won't be.

The odds of interference in this situation are "astronomically small," Meinrath says, in the same realm as worries that "a TV station could get hit by a meteorite." It might, and that would make for some compelling TV news footage, but it's hardly the sort of stuff policy makers worry about.